Method and device for error detection and diagnosis in a knock sensor

ABSTRACT

A method and a device for fault detection or diagnosis are described which are used on a multi-cylinder internal combustion engine with a knock sensor. For diagnostic purposes an instantaneous reference value formed from the sensor signal is compared with predefinable upper or lower reference threshold levels. If the instantaneous reference value exceeds or falls below the thresholds to a predefinable extent, a malfunction is detected. The reference levels correspond, for example, to the background noise present in knock-free operation. The fault detection or diagnosis is performed only for the cylinder which emits the loudest noise or from which the loudest noise is recorded.

[0001] The present invention relates to a method and a device for fault detection or diagnosis in a knock sensor on a multi-cylinder internal combustion engine according to the definition of the species of the main claim.

BACKGROUND INFORMATION

[0002] It is known that in order for knocking in an internal combustion engine to be regulated, the knocking must be reliably detected, with such detection ensuring that malfunctions of a knock sensor are reliably detected. Such knock sensors are assigned, for example, to each cylinder of the internal combustion engine. They emit an output signal which indicates whether or not knocking has occurred in the cylinder concerned. Knocking involves instances of undesired combustion, which should be avoided.

[0003] A method for knock detection and for detection of faults in knock sensors is known, for example, from German PCT Patent 94/01041. In this known method, engine speed-dependent normalized reference levels are formed from the output signals of the knock sensors, the latter being, for example, structure-borne noise sensors. As a function of the normalized reference levels, engine speed-dependent reference threshold levels are formed, with one engine speed-dependent upper reference threshold level and one engine speed-dependent lower reference threshold level being stored as a characteristic curve in the memory of the internal combustion engine's controller. The reference levels are formed such that they correspond to the noise level of the internal combustion engine in knock-free operation. The reference levels are set with the aid of integration of the output signals of the knock sensors, with determination of the reference levels being carried out under conditions under which no knocking occurs, for example within a predefinable measuring window.

[0004] For the purpose of the actual fault detection, in the known method each normalized instantaneous reference value is compared with the two engine speed-dependent reference threshold levels. In the event of a deviation, which may be predefined, of the normalized instantaneous reference level, a malfunction is detected. For example, a normalized instantaneous reference level that is below the lower reference threshold level indicates a drop-out of the knock sensor, such that no signal can proceed from it to the evaluation device. If the upper threshold level is exceeded, a malfunction of the sensor is again assumed, and appropriate replacement measures are initiated.

[0005] In order to be able to maintain a safety margin relative to the high background noise level which is caused by clatter, gas flow into the knock sensor line, etc., diagnosis is carried out only beyond a certain engine speed threshold, at which the normalized reference level is distant enough from the background noise for a reliable verification to be carried out.

ADVANTAGES OF THE INVENTION

[0006] The method according to the present invention and the device according to the present invention for diagnosis of knock sensors have the advantage over the related art that the engine speed threshold for the performance of diagnosis can be set lower than in the known fault detection method, without the reliability of the fault detection suffering thereby. This brings the advantage that early diagnosis of a knock sensor fault can be made at low engine speeds, rendering it possible for appropriate replacement measures to be initiated earlier, in order to protect the internal combustion engine reliably against damage.

[0007] These advantages are obtained via a method or a device for fault detection or diagnosis having the features as described in the two independent claims. The description is based on an internal combustion engine that has only one knock sensor, which is situated at a predefinable point on the internal combustion engine or the engine block. Several knock sensors may also be provided, and in such case specific cylinders are assigned to each knock sensor. For fault detection or diagnosis purposes, only the signal of the loudest cylinder is used; in other words, the signal of the cylinder which is loudest at the location of the knock sensor. Determination of the cylinder in question takes place in operation or when the internal combustion engine is first placed into operation, with the loudest cylinder being determined advantageously by a means for maximum value selection or by comparison of the reference levels obtained for the individual cylinders.

[0008] Further advantages of the present invention are obtained by the measures described in the dependent claims, according to which the fault detection or diagnosis is carried out advantageously by means of a comparison of the instantaneous reference level of the loudest cylinder with an upper and a lower reference threshold level. The reference threshold levels are advantageously also formed as a function of the reference level of the loudest cylinder and in addition are engine speed-dependent. It is advantageous if the maximum value selection in operation takes place by activation of a switch-over device. For purposes of regulating the knocking, all cylinders continue to be covered.

DRAWING

[0009] An exemplary embodiment of the present invention is shown in the drawing and is explained in greater detail in the following description.

[0010]FIG. 1 shows a known device for knock detection which, with the additional features shown in FIG. 3, is also suitable for performing the method according to the present invention.

[0011]FIG. 2 shows upper and lower reference threshold levels plotted against the engine speed as an example.

[0012]FIG. 3 shows a device according to the present invention for knock sensor diagnosis or knock sensor fault detection by evaluation of the noise of the cylinder which generates the loudest noises at the location of the knock sensor and thus causes the highest output signal of the knock sensor.

DESCRIPTION

[0013]FIG. 1 shows a device for knock detection on an internal combustion engine, which is known from German PCT Patent 94/01041. Such a device, with appropriate additional features, is also able to perform the method of fault detection according to the present invention and to initiate replacement measures in the event that a malfunction is detected.

[0014] In the device for knock detection shown in FIG. 1, two knock sensors 11 a and 11 b are assigned to internal combustion engine 10 which is shown only schematically, the knock sensors being situated at predefinable points on the internal combustion engine. Knock sensors 11 a and 11 b supply signals S1 and S2 to an evaluation circuit 12 which is linked with a microprocessor or computer 13 via input E1. The computer is a part of a controller of the internal combustion engine, not shown in greater detail. Supplementary signals, for example an engine speed signal n, can be supplied to computer 13 via an additional input E2. In response to the received signals, computer 13 activates, for example, the ignition output stages 14 of the internal combustion engine and/or further devices such as injection valves, etc., of the internal combustion engine, for example by emitting appropriate signals at output A. The entire evaluation circuit can also be completely integrated in computer 13 of the internal combustion engine's controller.

[0015] Evaluation circuit 12 contains in the exemplary embodiment at least one amplifier 15 with an adjustable gain factor, to which output signals S1 and S2 provided by knock sensors 11 a and 11 b can be supplied alternately via a multiplexer 21. The signals are further processed in a downstream band pass filter 16 and a demodulation circuit 17, for example a rectifier, and are finally integrated in integrator 18. The integrated signals are, or the resultant integrator value is, labeled as KI. Integrator value KI is compared in a comparator 19 with a reference level; the result of the comparison indicates knocking if the value KI is higher than reference level Ref by predefinable amounts.

[0016] Comparator 19 or further comparing devices not shown in FIG. 1, to which signals KI and Ref are also supplied, are also used for fault detection or diagnosis. To this end, the instantaneous reference level or a normalized instantaneous reference level formed from it is compared with an upper and a lower reference threshold level, and faults are detected if this comparison shows that an instantaneous reference level is higher than the upper reference threshold level or is lower than the lower reference threshold level. The two reference threshold levels are engine speed-dependent characteristic curves, with the reference threshold levels increasing with rising engine speed, since the noise emitted by the cylinders also increases as the engine speed rises.

[0017] In order to increase reliability in fault detection it is also stipulated that the state of being higher than the upper reference threshold level value or lower than the lower reference threshold level value must be maintained for an applicable duration.

[0018] In order to be able to maintain a safety margin relative to the background noise level, the fault detection or diagnosis is carried out only above a certain minimum engine speed nG, at which the reference threshold level is distant enough from the background noise. Such a minimum reference level Refmin thus requires a minimum engine speed nG.

[0019] An example of the characteristic curve of an upper and a lower reference threshold level related to the engine speed n and a minimum reference level is shown in FIG. 2.

[0020] The formation of the reference levels from the output signal of a knock sensor and the determination of the reference threshold levels from prior reference levels are described in greater detail in German PCT Patent 94/01041, and consequently will not be discussed further here.

[0021] In the case of internal combustion engines having only a single knock sensor, the differences in the reference levels which are determined for specific cylinders are sometimes very large. The differences in noise level between the inner and the outer cylinders are considerable and cause different reference levels to be obtained for each cylinder. The differences in noise level arise out of the differing distances of the individual cylinders from the point of installation of the knock sensor and out of differing transmission pathways for the structure-borne noise. For example, water ducts or oil ducts in the engine block, i.e., in the block of the internal combustion engine may affect the transmission of structure-borne noise. As a general rule, the noises generated in different cylinders can also be different. By limiting, in accordance with the present invention, the diagnosis to the loudest cylinder or cylinders or to the loudest noise recorded for a specific cylinder, lower reference threshold level REFMIN is moved further away from the background noise level, which is constant. As a result, the diagnosis can be carried out starting from lower engine speeds than is the case with the known system, and earlier diagnosis of a knock sensor fault is made possible; this likewise means that replacement measures can be initiated earlier in order to protect the internal combustion engine from damage.

[0022] Selection of the loudest cylinder or of the loudest cylinder-specific noise and thus of the highest reference level takes place in operation or when the internal combustion engine is first placed into operation. To this end the noises occurring are measured and the loudest noise, in other words the noise belonging to the loudest cylinder, is selected. By comparing the cylinder-specific output signals of the knock sensor with one another, it is thus possible to determine the loudest cylinders in operation, or at the first placing into operation, or on the test bed. Such a maximum value selection can also take place on the basis of the reference values determined for specific cylinders.

[0023] Subsequently, the fault detection or diagnosis will be subsequently based on this cylinder. Knock detection, however, will continue to cover all cylinders.

[0024] In the exemplary embodiment according to FIG. 3, a single knock sensor 11 is assigned to internal combustion engine 10 having four cylinders 1, 2, 3 and 4. Knock sensor 11 emits an output signal, which is dependent on the noises occurring in the cylinders. Cylinder-specific reference levels REF1 to REF4 are formed from the output signal of knock sensor 11, with the formation of the reference levels taking place in accordance with the method described in German PCT Patent 94/01041. For fault detection or diagnosis purposes, only one reference level is passed on to two comparators 24 and 25 via a maximum value selection device 22. Which reference level is to be passed on can be determined, for example, in operation by actuation of a switch in the maximum value selection circuit. This actuation is indicated by arrow 23.

[0025] In comparators 24, 25 the instantaneous reference value of the loudest cylinder is compared with upper reference threshold level REFSO and/or lower reference threshold level REFSU. The two reference threshold levels REFSO and/or REFSU are formed as a function of the engine speed from the reference level of the loudest cylinder with, for example, prior reference levels of the same cylinder also being taken into account. The reference threshold levels are, for example, formed in blocks 28 and 29 as a function of the supplied engine speed nmot and stored in an electronic memory device.

[0026] If one of the two comparisons shows that an impermissible value of the instantaneous reference level is present, a corresponding high signal is supplied to OR gate 26, as a result of which a fault signal is created at the output. The fault signal can be displayed on a fault display 30 and/or suitable measures to act on the ignition etc. can be initiated, thus placing the internal combustion engine in a safe operational condition. A timer circuit 27 may, if appropriate, cause the fault display to be activated only if the fault persists for a predefinable duration.

[0027] The exemplary embodiment according to FIG. 3 may be a component of evaluation circuit 12 or of computer 13 of the internal combustion engine's controller or may be an addition to it. If appropriate, the two loudest cylinders may also be selected.

[0028] A safety factor may additionally be incorporated in the fault detection, if appropriate, to permit fault detection only when the instantaneous reference level of the loudest cylinder exceeds the upper reference threshold level multiplied by that factor or falls below the lower reference threshold level multiplied by such a safety factor. 

What is claimed is:
 1. A method of fault detection or diagnosis in a knock sensor which is assigned to the cylinders of a multi-cylinder internal combustion engine in a predefinable manner and which supplies an output signal dependent upon the noise recorded, wherein the fault detection or diagnosis takes place on the basis of the cylinder or cylinders having the highest noise generation, or takes place on the basis of the cylinder or cylinders which supplies or supply the loudest noise at the knock sensor, with a fault being detected if a noise-dependent instantaneous value exceeds at least one predefinable upper threshold level, or falls below a predefinable lower threshold value.
 2. The method of fault detection or diagnosis according to claim 1, wherein the selection of the loudest cylinder takes place when the internal combustion engine is first placed into operation.
 3. The method of fault detection or diagnosis according to claim 1, wherein the selection of the loudest cylinder takes place by measurement of the noise level on the test bed or by comparison of the output signals emitted from the individual cylinders or of reference values derived therefrom.
 4. The method of fault detection or diagnosis according to claim 1, 2 or 3, wherein the fault detection is carried out on the basis of reference levels which are formed from predefinable output signals of the knock sensor, and a fault is detected if an instantaneous reference level exceeds an upper reference threshold level or falls below a lower reference threshold level in a predefinable manner.
 5. The method of fault detection or diagnosis according to claim 4, wherein the reference threshold levels are formed from prior reference levels of the loudest cylinder.
 6. The method of fault detection or diagnosis according to one of the preceding claims, wherein fault detection occurs only when the reference level exceeds an upper reference threshold level multiplied by at least one predefinable factor determining a safety range or drops below a lower reference threshold level multiplied by a second factor.
 7. The method of fault detection or diagnosis according to one of the preceding claims, wherein a fault is detected only if the fault condition persists for longer than a predefinable applicable period.
 8. The method of fault detection or diagnosis according to one of the preceding claims, wherein reference threshold levels are stored in an electronic memory device as engine speed-dependent characteristic curves.
 9. The method of fault detection or diagnosis according to one of the preceding claims, wherein when a fault is detected, replacement measures in order to ensure safe operation of the internal combustion engine and/or a display are initiated.
 10. A device for fault detection or diagnosis in a knock sensor which is assigned to the cylinders of a multi-cylinder internal combustion engine in a predefinable manner and supplies an output signal dependent upon the noise recorded, wherein the device includes at least means for detection of maximum values, which transmits for fault detection purposes only those signals which come from the cylinders having the highest noise generation or signals from the cylinder which supplies the loudest noise at the knock sensor.
 11. A device for fault detection or diagnosis in a knock sensor which is assigned to the cylinders of a multi-cylinder internal combustion engine in a predefinable manner, wherein the device is used to implement at least one of the methods according to one of the claims 1 to
 9. 